Like young birds that are leaving their nests for good, a few stars are taking wing from their galactic nest. They’re moving so fast that they’ll eventually escape from the Milky Way and head into the vastness of intergalactic space.

They’re known as “hypervelocity” stars. They’re moving more than a million miles per hour faster than the other stars in the galaxy. And they seem to come in two varieties. One consists of hot, heavy stars that were kicked out of the galaxy’s core. The other consists of stars that are similar to the Sun residing away from the core.

The first group was discovered almost a decade ago. These stars originally had stellar companions. These systems passed close to the supermassive black hole at the galaxy’s heart. A gravitational dance resulted in one of the stars being captured by the black hole, with the other getting a kick powerful enough to propel it out of the galaxy.

The second group was discovered just last year. A team of astronomers looked at a survey of hundreds of thousands of stars. The study revealed about 20 stars moving fast enough to escape the galaxy. Unlike the other group, though, these stars appeared to come from the galaxy’s disk, well outside the core. All of the stars are similar to the Sun.

There’s no obvious way for these stars to get a strong enough kick to leave the galaxy behind. That leaves the astronomers with a mystery to solve — the mystery of the runaway stars.